1. Field of the Invention
The present invention relates to drive control devices and image forming apparatuses such as a facsimile machine, a printer, a copy machine, etc, which uses a rotating object and, more particularly, to an image forming apparatus using an intermediate transfer member, which transfers a visible image on an image carrier to a movable object at a position where the image carrier opposes the movable object.
2. Description of the Related Art
There is known an image forming apparatus, especially a color image forming apparatus, which has a plurality of development units and an intermediate transfer belt. In an image forming apparatus of this type, it is known that a fluctuation in a conveyance speed of the intermediate transfer belt causes a color shift in a color image. Such a fluctuation in a conveyance speed may be caused by a fluctuation in rotation of a drive roller, which drives the intermediate transfer belt, due to eccentricity of the drive roller, thermal expansion of the drive roller, a load to the intermediate transfer belt during conveyance of a recording medium, a load to the intermediate transfer belt in a primary transfer bias of a photosensitive drum, or the like. One of causes of such a color shift is in that a plurality of color toner images are shifted from each other when overlapping each color image on the intermediate transfer belt. In order to eliminate such a color shift, there are suggested several methods to reduce a fluctuation in a speed of the intermediate transfer belt. As one of the methods, there is a method to correct a rotation fluctuation of a drive roller by using a rotary encoder provided to an idle roller of the intermediate transfer belt for detecting an angular speed. The rotary encoder used for this method comprises, for example, a disk concentrically provided to a rotational shaft of the idle roller, and a transmission-type photo-interrupter sandwiching the disk. The disk is provided with many slits arranged radially. The photo-interrupter detects a light passing through the slits so as to detect a pulse time of a pulse signal generated by the detection of the light. A conveyance speed of the intermediate transfer belt is calculated based on the detected value so as to perform a feedback control of the rotation of the drive roller.
As an image forming apparatus using the above-mentioned encoder, there is known an invention disclosed in Patent Document 1 or Patent Document 2. Patent Document 1 discloses a technique to eliminate a belt moving speed fluctuation due to eccentricity of a roller without using a filter. According to this technique, angular velocity information of an idle roller detected from a pulse signal from an encoder is stored in a first memory over one rotation period of a drive roller. Then, a speed detection error component due to eccentricity of the idle roller is extracted by canceling a speed fluctuation component due to eccentricity of the drive roller by operation of an operation circuit in the angular velocity information stored in the first memory, and the extracted speed detection error component is stored in a second memory. During image formation, a difference circuit acquires a difference between the angular velocity information of the idle roller detected from the pulse signal from the encoder and the speed detection error component stored in the second memory. Then, based on the difference data, a comparing circuit outputs a control signal to a motor driver so as to control the belt moving speed.
Patent Document 2 discloses an image forming apparatus capable of providing an excellent control result even if an encoder having a physically low resolution is used. The image forming apparatus comprises an image forming means for forming an image on a recording paper by forming a latent image on a photosensitive member by exposure and developing, transferring and fixing the latent image, a moving means used for a transferring process, and a drive means for rotationally driving the moving means. Moving distance information or moving speed information of the moving means is detected by an encoder. A position deflection or a speed deflection is acquired from an output of the encoder so as to perform a predetermined operation on the position deflection or the speed deflection. A drive source is controlled based on the result of the operation. In this image forming apparatus, a count is carried out at a predetermined period T within an output section of the encoder.
Patent Document 1: Japanese Laid-Open Patent Application No. 2000-047547
Patent Document 2: Japanese Laid-Open Patent Application No. 2004-205717
However, if widths of the slits of the rotary encoder are not uniform and intervals of the slits are not uniform, variation in the slit width and variation in the slit interval are recognized as variation in the speed of the intermediate transfer belt. Thus, it is required to maintain processing accuracy and position accuracy at a high level when forming the slits, which drives up costs in manufacturing. It is very much difficult to fabricate the slits with completely uniform slit intervals.
Moreover, while determining an amount of error in intervals of slits (objects to be detected) provided radially, speed fluctuation due to various loads to the intermediate transfer belt may be included in the amount of error. Thus, the intervals of the objects to be detected must always correspond to the amount of error. That is, it is necessary to completely grasp at which position it is stopped between the objects to be detected even when the intermediate transfer belt stops. However, if a user removes the intermediate transfer unit and moves the intermediate transfer belt, the corresponding relationship therebetween goes out of order. Additionally, a noise may enter the detection signal, which indicates intervals of the objects to be detected while the intermediate transfer belt is rotated, and if an error occurs in detection of the objects to be detected, it is possible that the correspondence cannot be acquired.